Heat the oven to 350°F. Line two 6-cup muffin tins with parchment liners.
In a large bowl, whisk the mashed banana, eggs, milk, maple syrup, and vanilla extract until well combined and no large lumps of banana remain.
Add the rolled oats, sliced almonds, cinnamon, baking powder, and salt. Stir until everything is evenly coated. Let the mixture sit for 5 to 10 minutes so the oats can absorb some of the liquid.
Divide the mixture evenly between the 12 lined cups, filling each about two-thirds full. Scatter a few extra sliced almonds over the tops.
Bake for 25 to 30 minutes, until the tops are lightly golden and set. The centers should not jiggle when the pan is gently moved. A toothpick inserted into the center should come out without wet batter, though a few moist crumbs are fine.
Let the oatmeal cups cool in the pans for 10 minutes before lifting them out.
Notes
Let the mixture rest 5 to 10 minutes before filling the tins. The oats need that time to absorb liquid or the cups spread rather than mound.
Fill each cup about two-thirds full. Overfilled cups set at the edges and stay wet in the centers. Old-fashioned rolled oats give the best texture.
Quick oats work, but the cups will be softer.
Use bananas that are yellow with plenty of brown spots for the most sweetness and flavor.
Nut-free: swap the sliced almonds for pumpkin seeds or sunflower seeds.
Storage: airtight container in the refrigerator for 4 to 5 days. The oats firm up as they sit.
Freezing: cool completely, then freeze in a freezer-safe bag for up to 3 months. Thaw overnight in the refrigerator and reheat in the microwave, or reheat from frozen, allowing extra time.
Four flavors: divide the rested batter into four bowls and fold in blueberries, diced apple, mini chocolate chips, or pumpkin pie spice before filling the tin.